I took it two years ago. Unfortunately I failed. I had never jumped my horse with trot poles in front and I never really trotted to a fence, so that was an utter disaster. I should have prepared better. I think my horse and I were not having a very match up day. I ended up passing everything for my switch rides and failed everything when I was on my horse. One of my most embarrassing rides to date. I wanted to retake it last summer, but I fell and broke my ankle, so I didn't have the chance. Now I have a green horse who will not be able to do a C3 rating for a long time, so I guess I have lost my chance.
My advice to you is to be confident in your answers. As you are riding, think about what you like and do not like about each movement, that way you have some feedback to give to your rater(s). Also, during warm up and group riding time, don't spend all your time diddling around on the outside rail. Show off what you can do. If you aren't so good at something, try not to do it right in front of the judge. Save your best movements for when they have their eyes on you. Even when you do something that absolutely sucks, be sure to tell the rater what you liked about it first. They like to hear that you are confident (but not oblivious to the fact that you can and will make mistakes). That would be my best advice for you. Good luck! I'm sure you'll pass with flying colors.